Preparation, Structure And Properties Of Carbon-Encapsulated Iron Nanocrystals

Carbon-encapsulated metal nanocrystals (CEMNs) are a new kind of carbon/metal nanocomposite, in which graphite layers arrange around metal nanocrystals located in the center to form core-shell structure. The carbon layers provide the oxidation resistance of bare metal nanoparticles and prevent them from environment degradation, and can endow some metal nanoparticles with biocompatibility. CEMNs, especially the magnetic nanomaterials, possess special structures and properties, and thus might have important applications in areas such as high-density magnetic data storage, ferrofluids, microwave absorption materials, oxidation-reduction catalyst, handling materials of radioactive waste and bio-medical materials.For the preparation of CEMNs, the present techniques, such as the arc-discharge technique, chemical vapor deposition, liquid-impregnating carbonization of non-graphitizing carbon and etc., show some disadvantages, e.g., the morphology and the structure of the resulting nanocrystals are not easy to control, and it is difficult to produce high-purity nanocrystals in large quantities. While here we proposed a novel method for the preparation of CEMNs by co-carbonization of aromatics with metal compounds, i.e., co-pyrolysis method. It featured in simplicity, low temperature, good controllability and high yield.The research focused on the preparation of CEMNs by co-pyrolysis of a refined aromatic heavy oil and ferrocene. The effect of the synthesis parameters on the formation and transformation of CEMNs were investigated in detail. The morphology, structure and particular properties of the product were characterized via TEM, HREM, XRD, SEM, TG/DSC and VSM measurements, and the relationship between them and synthesis parameters were studied to realize the controllable preparation of the product. The formation mechanism and thermo-transformation behavior of CEMNs were elucidated. In addition, the oxidation and further carbonization treatments on CEMNs were carried out and some interesting phenomenon and new results were obtained, which will provide a novel strategy for the research and development of various carbon nanomaterials.The results show that large amount of carbon-encapsulated iron nanoparticles (CEMPs) and nanorods (CEMRs) were obtained by co-pyrolysis of an aromatic heavy oil and ferrocene. With the increase of ferrocene content from 2 wt. % to 45 wt. % under the reaction temperature of 480℃, the morphology of the product had a transformation from CEMPs towards CEMRs. While with the increase of ferrocene content from 30 wt. % to 120 wt. % at 450℃, the morphology of the product were transformed from CEMRs to CEMPs. And the same morphology transformation took place with the enhancement of reaction temperature or the elongation of soaking time. Meanwhile, with the increase of ferrocene content, the encapsulated metal core in the product were changed from singleα-Fe to iron carbides (Fe₃C), and the carbon shells were turned from amorphous into turbostratic structure.By the analysis of raw feedstock and the investigation of the relationship among the morphology, structure of CEMNs and the synthetic parameters, it was concluded that the formation of CEMNs by co-pyrolysis method was based on the principle of condensation and polymerization in a vapor-liquid bi-phase by the aid of catalysis under pressure, and the catalysis of iron nanoparticles was based on the dissolution-precipitation mechanism. It is assumed that two essential factors must be met for the formation of the nanorods, including the sufficient amounts of iron clusters and moderate reaction activity in the system.By carbonization at 1000℃under N₂ atmosphere, the size of CEMNs became larger and the carbon shells were changed from the disordered into graphitic structure under the catalysis of nano-sized iron core. At the same time a large amount of hollow onion-like carbon nanoparticles emerged due to the ejection of iron nanoparticles. After oxidation of as-grown nanorods at 250℃, large amounts of ferric nanoparticles diffused and ejected from carbon shells, leading to the formation of carbon nanotubes with several nanoparticles trapped inside, and short nanotubes with diameter of 30-50 nm and length of 150-350 nm were generated by further carbonization at low and high temperatures.Both as-grown CEMNs and the carbonized product had a high coercivity and a small ratio of remanent magnetization (Mr) to saturation magnetization (Ms). It is implied that CEMNs show a combined magnetic characteristic of ferromagnetic and paramagnetic materials.The above research afforded a novel and effective method for the preparation of various carbon nanomaterials. It can be seen that, via the selectivity of raw materials, the controllability of synthesis parameters and the post-oxidation and carbonization treatments, not only CEMNs including nanoparticles and nanorods but also empty carbon onions and short carbon nanotubes were obtained by co-pyrolysis method. Moreover, it had a great significance for the promotion of the practical applications of CEMNs and for the synthesis, assembly and construction of carbon nanotubes and related nanostructures.